Modern devices can be susceptible to fault conditions, which may result in component and system damage. For example, during a drive operation of an optical drive such as a CD player, a CD/DVD combo player, a HD DVD player, or a Blu-ray player, the laser read power must be kept below a threshold, above which damage could occur to the laser. If the laser read power exceeds the threshold, the read laser may be damaged.
Typically, each device has built-in safety protection. In the above example, an optical drive typically includes an automatic power control loop. In addition, an optical drive may include a mechanism to shut down the read laser when the laser read power exceeds the threshold. The automatic power control loop can be disabled by an analog comparator circuit that continuously compares a forward sense voltage to the threshold. If the comparator detects a laser fault condition, the laser is automatically powered down, and an interrupt request is issued to notify device firmware of the fault condition.
However, a problem arises if a computer chip controlling the protection comparator erroneously sets the threshold to an inappropriate value. As a result, the device's built-in safety protection may not be invoked, even though the device is operating at an unsafe power level, because the power is under the erroneously-set threshold.
Moreover, the widespread use of firmware makes devices more susceptible to fault conditions. Firmware is a computer program embedded in a hardware component such as a microcontroller. A user may upload firmware onto existing hardware and update the firmware after the device is manufactured, either electronically, or by replacing a storage medium such as a socketed memory chip.
In optical drives, for example, built-in safety protection, such as an automatic power control loop, may typically be implemented in firmware. Therefore, it is conceivable that a laser fault condition could result if the firmware erroneously writes an inappropriate value to one or more of the register fields that determine the value of the current sent to the laser diode driver (LDD). One reason for this potential occurrence is a general firmware failure condition so severe as to require a power-on reset. Less severe firmware error conditions are also possible.
Notwithstanding any firmware errors, CPU failure, clocking errors, or other unexpected errors, the device must have a fault protection mechanism to protect the integrity of the threshold field. For example, in an optical drive, once the forward sense threshold value and other protected parameters have been set, the firmware cannot re-program them during normal drive operation, and the parameters may not be overwritten by other means. The parameters are thus protected in the sense that they may be programmed only once after each power-on operation.
Accordingly, it would be desirable to have additional methods and systems for fault protection using a linear feedback shift register.